That's because everyone knows it will be Palin. The other posers know that as soon as they start seriously posing, the sooner they will have to face reality, and they are putting that off for obvious reasons (aka because they are posers).
I read 538's analysis of the primaries, and it showed that Palin could compete pretty well in there, but that scenario didn't include Huckabee, who would siphon votes and delegates from her. Do you have any of those Iowa numbers, by the way? Not doubting you, I just want to see specifically how she's doing.